Cathodic protection of tank bottoms



March 16, 1954 s. P. EWING CATHODIC PROTECTION OF TANK BOTTOMS 2 Sheets-Sheet 1 Filed NOV. 15, 1952 0 D E 000 R U m -w H o oo 0O 0 k. G. I L 0 w .w

SPAN IN FEET FIGURE "2 I0 SPAN IN FEET FIGURE"? MN 7. m 0 i.. T 3 W w E7 U. +L W B March 16, 1954 s. P. EWING CATHODIC PROTECTION OF TANK BOTTOMS Filed Nov. 15, 1952 2 Sheets-Sheet 2 O m w v N O mN m M40: 9E IFnEQ EFFICIENCY-Z,

T. N p R er V0 H OQ W CW7 Pfl Patented Mar. 16, 1954 CATHODIC PROTECTION OF TANK BOTTOM Scott P. Ewing, Tulsa, okla, assignor to Standard Oil Development Company, a corporation of Delaware Application November 15, 1952, Serial No. 320,660

The present invention is concerned with an improved method of cathodically protecting a tank,

bottom from galvanic corrosion. The invention is more particularly concerned with an improved method of determining soil resistivities adjacent to tanks for the purpose of determining the cur rent required for cathodic protection of such tanks. In accordance with the specific adaptation of the present invention a plurality of electrodes are utilized at critically spaced distances from the tank which distances are a. function of the diameter of the tank bottom.

It is well known in the art to protect metallic tank bottoms from galvanic corrosion by various procedures. For instance, a widely used method for preventing corrosion of buried or submerged metallic structures is to make the structure electrically cathodic to the surrounding medium by causing'current to flow from an external anode to the structure. Under these conditions corrosion is prevented providing the current density on every part of the metallic tank bottom structure exceeds a certain minimum which depends on the metal and the surrounding medium. In general when a steel tank bottom is exposed to salt water it is desirable to have a current density at the center of the tank in the range from about 4 to 5 milliamperes per sq. it. On the other hand in most soils a current density in the range from about 1 to 3 milliamperes per sq. ft. is sufficient.

Heretofore in the art it was known that the current density at every part of a metallic disc can be determined when the medium surrounding the disc is of uniform resistivity and there is no polarization of the disc. (Smythe, Static and Dynamic Electricity, McGraw Hill, p. 114 (1950)). For instance it is known that the point of lowest current density of a metallic disc is the center of the disc where the current density is the average current density on the entire disc.

However, many anomalies exist with respect to the surrounding earth medium on which a tank bottom is positioned. For instance it is well known that the resistivity of the earth is not uniform, there-being variations with depth as well as with horizontal direction. However the variations with depth are much greater than variations in the horizontal direction. Thus heretofore in the art if a current density was to be determined with respect to the center of a tank bottom it was either necessary to empty the tank and cut through the plates on the tank bottom or to tunnel under the tank or to guess at the current density at the center.

The present invention is thus particularly con- 2 Claims. (01. 32462) cerned with an improved method of determining the current density on the external central part of the tank bottom when a known current is caused to flow through the earth from an external anode to the tank. The present invention utilizes a plurality of critically spaced electrodes the spacing of which is a function of the diameter of the tank bottom.

The process of the present invention may be more fully appreciated by reference to the drawings illustrating embodiments of the same. Figures 1 and 2 illustrate resistivities measured in ohm ft. with regard to various spanning distances of electrodes. Figure 3 illustrates the efliciency of current distribution on a metallic disc as determined from various experimental runs, the emciency being defined as the ratio of the current density at the center to the average current density on the disc. for determining the current density at the center of the tank bottom in accordance with the present invention.

Referring specificallyto Figure 1 it is noted that as the span between the electrodes increases, the resistivity decreases. This simply means that as the span between electrodes increases, the average depth of the electrical flux extends into lower substrata of higher conductivity. Here there exists a condition where the upper thin stratum of the earths surface has a lower conductivity than Figures 1 and 2 thus show that for the purpose of the present invention the earth may be considered to consist of a more or less thin surface layer of one resistivity which is underlain to great depth by a substrata medium of another resistivity.

Referring specifically to Figure 3 a number of experimental tests were made employing a tank model which in one set of runs was positioned over a Bakelite substratum and in another set of runs over a copper substratum. The model was constructed so that it was possible to measure the applied current density at 'numerous points in eluding its center and thus determine the efficiency of current distribution. Measurements Figure 4 illustrates the means were also made to determine the apparent resistivities p1 and 92 for various water depths over Bakelite and copper substrata. The results of these tests are illustrated in Figure 3.

It is evident that when p2 equals ,0: the abscissa becomes zero and the efiiciency is 50% (that is uniform resistivity ithe 'earths substrata). However when theiresistivity of the lower substrate. increases the efiiciency will decrease below 50%. On the other hand if the resistivity of the earths substrata decreases the emciencywillrise above 50%. Referring specifically to Figure l the potential difierence E between two points at distances 1'1 and T2 from the-'center'of asm'all uniform medium of resistivity p is Ipd7 1 1"]; I fr 41r7' 41r E (1) If the limits are chosen so that E= when n f and where the medium 1 is changed to semiinflnitethen iota a b IP 1 I 721 a (3) Novr'theresistance I e-z (1-0-(2-3) n 270 01' p =27taR (4) Similar to the development indicated in Equatlon 2 it can be shown that the potential at a point locate'd a -distance 'aifrom the center of a 'conductingdisc of radius randdischarging aeurrentI is:

(Earth Conduction Effects in' Transmission' systems,E. D. Sande, D. Van NostraudCa, Inc., N. Y.;p.-44,'-l949.)

ltlshoflld r be noted that-1 this relationship holds true only in" the planeof the disc.

Now if the disc is substituted for one current electrode; in the fourelectrodemethod ofresistivltymeasurementas schematically i-llustra'ted 'by Flgure'4.

'I'hen thepotentlal drop from'thedisc to elec trode=2r can" beiexpressed as follows:

.- In "r p lg ur -ar tem) "zer' r-i'-b)' 41rr "that the'di'stanc'e b=UI3225r and the resistances .'.The above expression is true only in a medium of uniform resistivity. Since actual soil resistiv- I 'ities vary with depthg and coatings may be present spherical electrode dischargingcurrent I in "a on the tank bottom, R1 and R2 will in general be difi'erent. 'Itis"therefore convenient to define R1 and-Rain termsiof -two apparent resistivities as Thus-dais: evident that if the current density on-thecenter .of a-tank bottom. isthereby determined this 1 can be readily accomplished bythe positioning-of three external electrodes. The first electrode will be positioned .3225 tank diameters from the-tank. The secondelectrode will be positioned one tank diameter-fromthe firstelectrode while the third electrode will be positioned in a straight lineone tank diameterIfromthe second electrode.

The operation current is;applied between the tank and the third electrode. The potential change produced.- by. this current is measured betweenthe'tank andthe first electrode and between the first. and second electrode. These potentialchanges are designated as E2 and E1 respectively. "Theefiiciency is then. 'determined'in accordance. with the procedure described heretofore.

While the above. procedure has been described foi the determiningof current density onanoncoated disccr. tank bottom, it. is to beunderstood that "the. procedure is .equally applicable for the determiningof current density on. coated discs or tank bottoms. Thus. the method isv very applicable for efiicientlydetermining the-resistance of the coating.

The present invention may be more fully understood by the "following example illustrating the same:

"Example f Due tosoil' conditions it was necessary toapply 5 milliamperes per sq. it." at thetcenter of the .bottoms-of tanksA'and B.

With respect-to tank 'A it-was determined that From Figure 3: it is evident that the efiiciency was 20%.

' With respect to: tank 18. it was. determined that 2. 1 z-F'Ri From Figure 3 it is evident that the efilciency with respect to tank B was v The bottom of .each tank had. an. area of approximately. 10,000 sq.l it., meaning that. approximately 50,000. milllamperes wouldhave tobelsupplied, assuming the current density to be uniform and the efliciency to be was .3

Thus it is evident that with respect to tank A having a 20% efiiciency it is necessary to supply 250 amperes While with respect to tank B With 80% efficiency, it is necessary to supply 62.5 amperes.

Having described the invention, it is claimed:

1. Improved process for determining the current necessary to be supplied to a metallic disc positioned on any type of earth surface to secure a predetermined current density at the center of said disc, which comprises positioning on said earth surface an initial electrode a distance of .3225 diameters from said disc, positioning a second electrode one disc diameter from said initial electrode, positioning a third electrode one disc diameter away from said second electrode, all

electrodes being on the projection of a tank radius, causing current to flow between said structure and between said outer electrode, measuring the potential change produced between said disc and said first electrode, and between said first and said second electrode, designating these resistances as R2 and R1, respectively and determining the relationship of (R2R1) over (R2}R1) and determining the efiiciency, whereby the total amount of current necessary to be furnished is determined.

2. Processes defined by claim 1 wherein said disc comprises a tank bottom.

SCOTT P. EWING.

No references cited. 

